Electricity

• What are the two types of current

  Alternating current- current is constantly changing direction. The UKs main supply is an alternating current at 230v and frequency 50Hz

  Direct current- current that is always flowing in the same direction. It is created by voltage and the cells and battery supply the direct current.
• Power equation


  P=VI

  Power= potenetial difference x current
• What does a step up transformer do?

  Potential difference is increased and current is decreased

  This is so energy lost is decreased
• What does a step down transformer do?

  decreases the voltage so it is safe to use in homes by increasing the current
• Energy transferred equatios


  E=QV

  Energy transferred= charge flow x potential difference

  energy transferred= power x time
• National grid


  A system of cables and transformers that links power stations to consumers
• Series circuits


  The different components are connected in a line

  If you remove or disconnect one component the circuit is broken

  Potential difference is shared everywhere

  current is the same everywhere

  resistance adds up

  cell potential difference adds up
• Parallel circuits


  each componented is seperately connected to the +ve and -ve of the supply

  if you remove or disconnect one of them, it will hardly affect the others at all

  potential difference is the same across all components

  current is shared between branches

  adding a resistor in parallel reduces the total resistance
• Static electricity


  When certain insulating materials are rubbed together and negatively charged electrons are scraped off one and dumped on the other

  this will leave the materials electrically charged

  one with a + charge and one with a - charge

  only the electrons move
• Three wires in cables

  All have a core of copper and a coloured plastic coating

  1. Live wire- brown, provides alternating potential difference from mains power supply
  2. neutral wire- blue, completes the circuit- when appliance is running normally, current flows through the live and neutral wires. It is around 0 volts
  3. Earth wire- green and yellow, for protecting the wiring and for safety- stops the appliance from becoming live. Provides a low resistance path to the ground
• Electric current


  The flow of electrical charge
• Equation linking charge, current and time

  1. Q = I t

  2. Charge (Coulombs)

  3. Current (Amperes)

  4. Time (Seconds)
• Current is the same at all points in a closed loop
• Factors current in a circuit depends on

  • 1. Potential Difference (V)
  • 2. Resistance (R)
• Equation to calculate potential difference if current and resistance are known

  1. V = I R

  2. Potential Difference (V)

  3. Current (A)

  4. Resistance (Ω)
• Ohmic Conductor


  • A conductor for which current and potential difference are directly proportional
  • Resistance remains constant as current changes
  • Temperature must be constant
• Components for which resistance is not constant as current changes


  • Lamps
  • Diodes
  • Thermistors
  • Light Dependant Resistors (LDRs)
• Resistance of a filament lamp as temperature increases


  • Resistance increases
  • Ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow
• Current flow through a diode


  • The current only flows in one direction
  • Resistance is very high in the other direction, preventing current flow
• Resistance of a thermistor as temperature increases

  The thermistor's resistance decreases
• When a thermistor may be used

  • In a thermostat to turn a heater on below a certain temperature
  • In a freezer to turn on a cooler when the temperature becomes too high
• Resistance of a LDR as light intensity decreases


  The LDR's resistance increases
• Application for a LDR

  • Street lights often use LDRs
  • When light levels become too low, the light gains sufficient current to turn on
• Series


  Same loop
• Parallel


  Adjacent loop
• How the potential difference across two components varies when connected in series and parallel

  1. Series: Total P.D is shared between each component

  2. Parallel: P.D across each component is the same
• Total resistance of two resistors connected in parallel


  Less than the smallest of the two individual resistances
• Total resistance of two resistors connected in series

  Equal to the sum of the two individual resistances
• Current in a series circuit

  • The same at all positions since the charge only has one path to flow through
• Current in a parallel circuit

  • Shared between the different branches, when the charge reaches a junction it splits
• How to connect an ammeter in a circuit to measure current

  Ammeters should be connected in series with the component that they are measuring current through
• How to connect a voltmeter in a circuit to measure potential difference

  Voltmeters should be connected in parallel to the component that they are measuring the potential difference of
• Connecting lamps in parallel


  If one lamp blows, the rest will be unaffected and can still receive current (i.e. the circuit is still complete)
• Mains electricity


  a.c supply
• a.c.


  Alternating Current
• d.c.


  Direct Current
• Alternating current


  Current that continuously changes direction at a specific frequency
• Direct current


  One directional current flow
• Mains electricity in the UK is an a.c supply
• The frequency of the UK mains electricity supply is 50 Hz